Polysulfide impregnation of lignocellulosic materials in a continuous digester

ABSTRACT

A continuous process for protecting lignocellulosic materials, such as wood chips, against degradation in continuous alkaline pulping processes which includes continuously impregnating lignocellulosic materials with an aqueous polysulfide ion containing solution at a temperature below 140*C. and an ambient pH of 11.0 to 13.0, withdrawing the excess solution, treating with a hydroxyl ion containing solution, preferably kraft white liquor, whereby the aldehyde end groups of the lignocellulosic materials are oxidized to carboxyl groups, and subsequently continuously digesting. The polysulfide ion-containing solution is an aqueous polysulfide liquor produced by continuously admixing molten sulfur and hydroxyl ion depleted sulfide liquor at a pH between 10 and 12 resulting from stripping of hydroxyl ions from white liquor by the lignocellulosic materials.

United States Patent 1 1] 3,874,991

K pp 1 Apr. 1, 1975 [5 POLYSULFIDE IMPREGNATION OF 3,723,242 3/1973 Barker [62/42 x LIGNOCELLULOSIC MATERIALS IN A FOREIGN PATENTS OR APPLICATIONS CONTINUOUS DIGESTER 106.865 6/1965 Norway Primary Examiner-S. Leon Bashore Assistant Examiner-Arthur L. Corbin [5 7] ABSTRACT A continuous process for protecting lignocellulosic materials, such as wood chips, against degradation in continuous alkaline pulping processes which includes continuously impregnating lignocellulosic materials with an aqueous polysulfide ion containing solution at a temperature below 140C. and an ambient pH of 11.0 to 13.0, withdrawing the excess solution, treating with a hydroxyl ion containing solution, preferably kraft white liquor, whereby the aldehyde end groups of the lignocellulosic materials are oxidized to carboxyl groups, and subsequently continuously digesting. The polysulfide ion-containing solution is an aqueous polysulfide liquor produced by continuously admixing molten sulfur and hydroxyl ion depleted sulfide liquor at a pH between 10 and 12 resulting from stripping of hydroxyl ions from white liquor by the lignocellulosic materials.

4 Claims, 1 Drawing Figure wooo cums (STEAMED) POLYSULFIDE 10w SOLUTION PREPARATION RECYCLE (ems-150%) IMPREGNATING l ZONE 1 SPENT Isms-140%) "OMEN I POLYSULEIDE SOLUTION I HY ROXYL-ION DEFLETED SULFER WH1TE LIQUOR lpH 10-121 I norms 2on5 WHlTE -l LIQUOR wifi 190c-|40 C) I u UOR J B P'E L PROTECTED CHIPS 3 WHITE uouoa 1121111 L RECoVERY CONVENTIONAL commuous DIGESTING zone (ABOVE 5001 BLACK LIQUOR PULP WASHING ZONE R I'IEIIIEIJAHI I ma 3.874.991

WOOD CHIPS (STEAMED) POLYSULFIDE ION SOLUTION PREPARATION RECYCLE (90C-l50C) "2 i IMPREGNATING L ZONE I SPENT (90CI40C) MOUEN POLYSULFIDE SOLUTION HYDROXYL-ION DEPLETED SULFER WHITE LIQUOR (pH IO-IZ) WH'TE LIQUOR I TREATING ZONE WHITE LIQUOR Q wfimLE n-MC i LIQU OR I JQWWL PROTECTED CHIPS a WHITE LIQUOR CONVENTIONAL CHEMICAL RECOVERY CONVENHONAL STEAM CONTINUOUS DIGESTING ZONE (ABOVE I50cI BLACK LIQUOR PULP H20 WASHING ZONE PULP ' INVENTOR PEDER J. KLEPPE ATTORNEY POLYSULFIDE IMPREGNATION OF LIGNOCELLULOSIC MATERIALS IN A CONTINUOUS DIGESTER CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my co pending application Ser. No. 754,893, filed Aug. 23, 1968, now abandoned, and a continuation of Ser. No. 96,164, filed Dec. 8, 1970, now abandoned;

BACKGROUND OF THE INVENTION This invention relates to protection of lignocellulosic materials, particularly wood, against alkaline attack during continuous digesting. More particularly, this invention relates to a process for inceasing pulp yield by impregnating wood chips with a polysulfide solution and treating with a hydroxyl ion solution.

Lignocellulosic materials (such as wood from coniferous and deciduous species of trees, the stalks of bamboo and reeds, bagasse, and cotton fibers) principally contain lignin, cellulose, and hemicelluloses. Hemicelluloses, such as glucomannan chains in softwood or glucurono-xylan chains in hardwood, are principally located in the cell walls of the fibers but lignin is located both in cell walls and in the inter-fiber spaces. In chemical processes for utilizing lignocellulosic materials, lignin in the inter-fiber spaces must be dissolved, with as little damage as possible to the fibers, so that the fibers are readily separable. Hemicelluloses can be retained on the surface of the fibers, in which case they can influence the bonding properties of the fibers, and they are also retained in the cell wall of the fibers, improving the flexibility and thereby the papermaking properties of the pulp. It is consequently desirable to retain the hemicelluloses in order to produce paper products having superior strength characteristics.

In alkaline pulping processes, a large portion of the hemicelluloses are usually dissolved in addition to lignin, particularly if the digestion is continued long enough to dissolve almost all of the lignin. The soda pulping process, which requires lengthy treatment, causes nearly all of the hemicelluloses to be dissolved and the cellulose chains to be broken at intervals, so that fibers are weakened and relatively little hemicellulose is available for bonding. Kraft and neutral sulfite pulping liquors are more specific toward lignin so that the cellulose is attacked very little, but hemicelluloses are attacked to a high degree. The dry weight of washed fibers which are recovered after pulping is generally reported as a percentage of the weight of dry lignocellulosic material which was charged to the digestion process. This percentage is termed yield. Any decrease in yield caused by loss of lignocellulosic materials is undesirable in papermaking.

1n alkaline pulping, three types of carbohydratedissolving reactions are important: (a) simple dissolution of polysaccharides having very low molecular weight, (b) successive splitting of sugar units from the reducing ends of the lignocellulosic chains by means of a peeling" reaction, and (c) splitting of the lignocellulosic chains into shorter fragments by hydrolysis of the ether linkages between the carbohydrate units. Reaction (a) is unavoidable because the very low molecular-weight polysaccharides are easily dissolved in aqueous alkali and in some instances even in water. Reaction (b) occurs as lignocellulosic material is being heated in contact with a pulping solution. It is very extensive above C. at a pH above 12 and continues until about sixty carbohydrate units have been successively peeled from the end of a lignocellulosic chain; it terminates because of autostabilization by formation of metasaccharinic acids. Reaction (c) is of importance at high temperatures (above 150C.) but has serious effects because an aldehydic end group is formed every time a chain is split. Thus, reaction (b), the successive peeling of terminal carbohydrate units, can start again.

In US. Pat. No. 2,944,928 Kibrick et al. discloses significantly increased yields for pulping with sulfide polymers or alkali polysulfides. Landmark et a1. concluded in 48 Tappi, No. 5 (1965), that polysulfide cannot be recovered as such in a kraft recovery system but only in its reduced form as sulfide; consequently, added sulfur or polysulfide is cyclically accumulated as sulfide in the system. In Norwegian US. Pat. No. 106,865, Landmark et al. describes a two-stage polysulfide pulping process in which wood is impregnated with a polysulfide-containing liquor at a temperature below C. and then cooked with a conventional kraft cooking 1iquor, whereby yields are increased.

It is the general object of thisinvention to retain hemicelluloses in the fibers during digestion by protecting the hemicelluloses against degradation; consequently, resulting in an increasein yield of pulp. Another object of this invention is to place a maximum amount of polysulfide ions in relation to the amount of added sulfur within the chips before digesting. Still another object is to main pH below the lignocellulosic material degradation pH while the protective solution is being absorbed. A further object is to utilize the hydroxyl-ion stripping capacity of lignocellulosic material to furnish a base solution, having a sufficiently low effective-alkali content, from which the polysulfide ion containing solution can be prepared. It is an additional object to locate elementary sulfur in the form of polysulfide ions in the interior of the chips while the surrounding pH is low whereby the polysulfide ion can stabilize the hemicelluloses when the pH is raised by subsequent immersion of the impregnated chips in strong white liquor. Yet another object is to obtain the increased yield of polysulfide pulping while operating the recovery process at sulfidity levels which are normally for a kraft process. Other objects and advantages of the invention will become apparent from the following description taken in connection with the drawing.

SUMMARY OF THE INVENTION This invention depends upon the substitution of a less reactive group for the aldehydic end group on the terminal sugar unit of a lignocellulosic chain as a means of stopping the peeling reaction. Polysulfides, when dissolved in hydroxyl ion depleted kraft white liquor to form a protective solution, aids in stabilizing the carbohydrates in wood against alkaline degradation during pulping, thus increasing the yield of pulp. Wood chips are mixed with a polysulfide ion containing solution at a temperature below normal cooking temperatures, held at this temperature while adsorption of polysulfide ions occurs, separated from the spent polysulfide solution, mixed with kraft white liquor at a second temperature which may be higher than the first temperature, held at this temperature while adsorption of hydroxyl ions occurs, e.g., the pH drops to from 10 to 12, separated from the hydroxyl ion depletedkraft white liquor which then is used to form fresh polysulfide ion containing solution. Spent polysulfide liquor and hydroxyl ion depleted white liquor at a temperature between 90C. and 150C., are mixed with low-viscosity molten sulfur under high-shear conditions to make fresh polysulfide solution for admixture with steamed chips.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is a flow sheet illustrating a preferred method which can be employed to practice the continuous process of this invention.

DETAILED DECRIPTION OF THE INVENTION The invention will best be understood by reference to the following detailed description and examples taken in conjunction with the accompanying drawing. The process of this invention performs high hemicellulose preserving of pulp and at the same time dissolves the added sulfur in the digesting liquor with less than 10% disproportion. The process, which is applicable only to continuous pulping processes, employs the following steps: (1 all of the white liquor needed for the digesting, including hydroxyl-ion depleted white liquor (approximately l70200% on D. chips weight), is added to the impregnating and treating zones and a low alkaline liquor (pl-l -12) is obtained from the treating zone and is combined with spent polysulfide liquor to form the spent polysulfide-hydroxyl ion depleted white liquor solution (pH 10-12) used to make-up fresh impregnating solution, (2) to the make-up solution 0.5 2.0% molten elemental sulfur is mixed at a temperature of 90C. 150C., and (3) the fresh polysulfide ion containing impregnating solution is contacted with the chips at a temperature below digestion temperatures, 90C. to 140C, preferably 100C. to 130C. This polysulfide ion impregnation and hydroxyl ion treatment of chips prior to digestion stabilizes the hemicellulose and cellulose against degradation. Stabilization is accomplished by the reaction of the polysulfide ions and hydroxyl ions to oxidize the aldehyde end groups ofthe cellulose and hemicellulose chains to stable carboxylic' groups. Utilization of the hydroxyl ion depleted white liquor to make-up fresh impregnating solution is the important point of this invention because the hydroxyl ion depleted white liquor is at a relatively low pH (IO-l2) which enables the polysulfide ions to be more efficiently utilized because they are not as easily degraded at elevated temperatures.

Referring now to the process as set forth in the drawing, wood chips, preferably steamed, are fed into an impregnating zone and mixed with an aqueous impregnating solution of polysulfide liquor. It is preferable to steam the wood chips prior to impregnating to enable the impregnating liquor to make good and rapid contact throughout the chips. The steamed chips normally enter the impregnating zone at a temperature between about 100C. l 10C. As the chips enter the impregnating zone, they are fed through a high pressure feeder and mixed with an aqueous polysulfide ioncontaining solution which is at a temperature between about 90C. 140C. and an ambient relatively low pH of between 1 l and 13, preferably between a pH of l 1.7 and 12.7 so that the polysulfide ions are not destroyed by hydroxyl ions.

mally carried out at digesting pressures, but alternatively it may be carried out at atmospheric pressure. Upon passing through the impregnating zone, about minutes, the excess spent polysulfide solution is withdrawn from the impregnated chips. One portion of the spent polysulfide solution is combined with water and white liquor recirculated to the impregnating zone. Such controls may be provided as are necessary to determine the proper amount of polysulfide solution, water and white liquor necessary to maintain the proper pH and temperature in the impregnating zone. The other portion of the spent polysulfide solution passes through a pH meter and circulation valve set at a preselected flow rate which determines the circulation factor. The pH information is used to control the amount of spent polysulfide solution in each portion and to make adjustments in the recirculated solution. The circulation factor is selected in accordance with the raw materials to be used and the equipment design. The flow of spent polysulfide solution is substantially detoured when intercepted by and mixed with the relatively high-volume flow of heated recirculation liquor.

To minimize downward flow of spent polysulfide liquor, the downwardmoving chips are also met in a treating zone by an upward flow of hot draft liquor. The amount of white liquor is dependent upon the amount of water in the steamed chips and the amount of polysulfide liquor which has been absorbed by the chips. If,

for example, the amount of absorbed hot white liquor is so large that it contains insufficient hydroxyl ions, additional white liquor is mixed with the recirculated polysulfide solution.

Most of the white liquor enters the treating zone after being combined with recycling white liquor. The combined streams of white liquor are heated and passes into the treating zone where the white liquor is split into an upward-moving stream and a downwardmoving stream, depending upon the amount of liquor which has been absorbed in the impregnating zone and the amount of water and white liquor added to the recirculating polysulfide solution. Hydroxyl ions are stripped from the upward-moving stream of white liquor, moving countercurrent to the polysulfide impregnated chips, so that the effective-alkali content of the white liquor has dropped significantly when the stream reaches the top of the treating zone and combines with the spent polysulfide solution.

The aqueous polysulfide ion-containing solution used in the impregnating zone is made up of the following components; (1) combined spent polysulfide solutionhydroxyl ion depleted white liquor, (2) molten sulfur, (3) water, and (4) white liquor, if necessary. The spent polysulfide solution at a pH of 10-l2, containing hydroxyl ion depleted white liquor, is mixed with molten sulfur at low viscosity at a temperature between C. and C. under conditions of high-shear mixing to produce an impregnating solution containing polysulfide ions. Water and white liquor are added where necessary to bring the pH to the desired range of between 11 and 13.

After leaving the treating zone, the chips and white liquor pass through a digesting zone where they are digested and washed using conventional procedures and apparatus for continuous digesters. Black liquor leaves after the digesting and washing zone and is conventionally converted to white liquor.

The ratio of impregnating solution to dry chips can vary tenfold, from one to ten, and may be expressed in terms of a circulation factor which is defined as the weight ratio of impregnating solution and dry chips which are mixed. 1f the circulation factor is approximately one, the chips and protective solution move at approximately the same speed. If the circulation factor is relatively high, such as five or more, the chips are rapidly carried to and into the digester and packed into the top thereof by the on-rushing protective solution which rapidly moves past the deposited chips. The circulation factor may be described as the number of round trips taken by the protective solution during a typical journey for the steamed chips which generally require only about one minute to reach the digester top and then need about fifteen to thirty minutes to pass through the impregnating zone.

It is necessary that at least a limited amount of stripping occur in order to remove any unused polysulfide liquor from the surfaces of the impregnated chips, because polysulfide atoms are rapidly changed to thiosulfate ions when contact by hot, strong alkaline solution, thus wasting both unused polysulfide and newly added alkali, according to the equation:

For example, 21% of the polysulfide ions in a polysulfide solution having an ambient pH of 13 to 14 are broken down in 30 minutes when the temperature is kept at 120C. If the pH is low, however, polysulfide atoms are relatively stable at high temperatures. For example, in a polysulfide solution having an ambient pH of 12, only a small portion of the polysulfide ions are broken down by alkali and reduced to thiosulfate ions at 120C.

Competition exists between the breakdown reactions of alkali upon the polysulfide ions and the stabilizing reactions of the polysulfide ions upon hemicelluloses. It is consequently desirable to site elemental sulfur, in the form of polysulfide ions, in the interior of the chips while the surrounding pH is low. The stabilizing activity of the polysulfides can then occur when the pH is raised by immersion of the impregnated chips in strong white liquor. Prior siting of the polysulfide ions is particularly important because hydroxyl ions migrate faster through wood than do polysulfide ions; thus, if both are present in large quantity at the same time, hydroxyl ions would preempt reaction sites in the chips and cause undesired peeling of carbohydrate chain.

Voids in the steam chips are substantially filled with water even though the chips may be able to absorb half as much again additional water. When mixed with the polysulfide-ion containing impregnating solution, an additional amount of liquid is absorbed, and the concentration of polysulfide ions becomes substantially uniform within and without the chips. Polysulfide-ion concentration drops as concentrations equalize within and without the chips. The hydroxyl ions however react when added even though the temperature is below the cooking temperature. Shortly before admixture of fresh white liquor, the hydroxyl-ion content of the liquor is substantially reduced if the circulation factor is low and is partially reduced if the circulation factor is high, but the polysulfide-ion content is simply equalized in either situation. Relative dilution of the polysulfide ioncontaining solution, for each cyclic use thereof, by

water in the steamed chips occurs inversely with the circulation factor.

The practice of this invention may clearly be seen in the following examples.

EXAMPLE 1 Southern pine chips were steamed for 30 minutes at C., driving out air and reducing the chip-solids content to approximately 45% on an ovendry basis. The steamed chips were treated with polysulfide protective solution, at a liquor-to-wood ratio of 143110 (weight basis), at 1 10C. and under psig pressure. The polysulfide-ion containing impregnating solution had a Na S content of 11.5 grams per liter, a polysulfide content of 7.5 grams per liter, and an effectivealkali content of 7.6 grams NaOH per liter. The effective alkali applied to the wood as 10.2% on an oven-dry weight basis. The analyses for remaining effective alkali and the calculations of consumed effective alkali at stated time intervals are as follows:

Effective Alkali Time of Treat- Content in Solution, Consumption *By calculation, allowing for dilution by the water in the steamed chips. The liquor absorbed equaled 1407: to 150% of the oven-dry wood weight.

The results show that 3.7% effective alkali as NaOH (on. CD. Wood) is consumed by treating wood at 1 10C. for 30 minutes with a polysulfide liquor having an initial effective alkali content of 7.05 g NaOH per liter (allowing for dilution) which diminished to 4.5 g/l. This means that 3.7% of dry wood weight has to be added as NaOH for maintaining an unchanged effective-alkali concentration during the impregnation. Assuming that the white liquor has a sulfidity of 30%, which is normal for modern kraft mills, the sulfide sulfur which has to be added with the white liquor for maintaining the effective alkali concentration is approximately 0.5% (on O.D. wood). This amount of sulfide sulfur will allow an addition of up to 2% sulfur (on wood), without exceeding the critical polysulfidezsulfur ratio of 4:1 (Na S The relatively low pH in terms of effective alkal may be ascertained from the table shown in Example 5.

EXAMPLE 2 Another run was made with the same steamed pinewood chips at approximately 45% solids content. The liquor-to-dry wood ratio was also l4.3:l.0, but the temperature and pressure were maintained at 80C. and 25 psig. The polysulfide protective solution had an Na S content of 7.9 grams Na S per liter and an effectivealkali content of 7.9 grams NaOH per liter so that the effective alkali applied to the wood equaled 10.5% on an oven-dry weight basis. The analyses for remaining effective alkali and the calculations of consumed effective alkali at stated time intervals are as follows:

Effective Alkali Time of Treat- Content in Solution, Consumption By calculation. allowing for dilution by the water in the steamed chips. The liquor absorbed equaled 140% to 150% of the oven-dry wood weight.

The results show that 3% effective alkali as NaOH on D. wood is consumed at 80C. if wood is treated for 30 minutes with a polysulfide solution having an effective alkaliconcentration of 7.3 g. NaOH per liter (allowing for dilution) which diminished to 5.3 g NaOH/l.

ln order to maintatin an unchanged effective alkali concentration by addition of while liquor of 30% sulfidity, approximately 0.4% sulfide sulfur must be added. This addition allows for a sulfur addition of up to 1.6% (on wood) before exceeding the critical polysulfidezsulfide ratio of 4:1.

EXAMPLE 3 A third run was made with the same steamed pinewood chips at approximately 45% solids content. The liquor-to-dry-wood ratio was l3.5:l.0, and the temperature and pressure were maintained at 100C. and 120 psig. The liquor absorbed equaled 145% of the ovendry wood weight. The polysulfide protective solution had an Na S content of 7.4 grams Na S per liter and an effective alkali content of 1.6 grams NaOH per liter so that the effective alkali applied to the wood equaled 2% on an oven-dry weight basis. The analyses for remaining effective alkali and the calculations of consumed effective alkali at stated time intervals are as follows:

Effective Alkali Time of Treat- Content in Solution, Consumption By calculation, allowing for dilution by the water in the steamed chips.

The results show that 1.7% effective alkali is consumed at 110C. by treating wood for 30 minutes at 1 C. with a polysulfide liquor having an initial effective-alkali concentration of 1.5 G NaOH per liter, after dilution, which diminished to 0.25 g NaOH/1. In order to maintain the effective-alkali concentration at an approximately constant level in the impregnation process by adding white liquor of 30% sulfidity, approximately 0.25% sulfide sulfurmust be added to the impregnation liquor. This amount allows a sulfur addition of up to 1% (on wood).

Examples 1 to 3 demonstrate practical conditions for the impregnation zone in continuous cooking according to this invention.

EXAMPLE 4 A polysulfide-ion containing impregnating solution was made by adding elemental sulfur to an alkaline solution containing 8.0 g NaOH/l as effective alkali and 12.2 g Na s/liter. The temperature of the solution was -84C., and good stirring of the solution was provided. The amount of sulfur added was 15 g/liter, which was completely dissolved after 15 minutes. Polysulfidesulfur analysis of the liquor after 15 minutes and after 3 hours storage at 93C. showed 15 g/liter polysulfide sulfur in each instance. This result demonstrated that of the added elemental sulfur was converted to polysulfide-sulfur and that the prepared polysulfidesulfur was stable at 93C.

EXAMPLE 5 Chips were treated at 1 10C. with a polysulfide protective solution containing 7.5 g polysulfide sulfur per liter. Samples of the liquor were taken at elasped time intervals and analyzed for effective alkali and pH. The results are given below.

pH Effective Alkali, gr/l EXAMPLE 6 To illustrate the improved yield increase obtained using the process of this invention, an impregnated cook was compared to a conventional polysulfide and conventional kraft cook. The samples of Southern pine chips 1500 grams) used were first steamed for 30 minutes at C.

Cook No. l, the polysulfide impregnated chip cook, was carried out by impregnating at 110C. with 18.8 1iters polysulfide liquor (circulation factor 12.5) containing 1.6 g/l effective alkali (as NaOH and 7.5 g/l polysulfide sulfur. The sulfide content of the liquor was 7.4 g Na S/l, and the initial pH was 12.2. The impregnation was performed for 60 minutes at -120 psig. The polysulfide liquor had beforehand been stripped for alkali at 1 10C. by chips. The liquor which had not been absorbed by the chips was then drained off. The amount of liquor absorbed by the chips equaled of O.D.chip weight. Polysulfide sulfur absorbed by the chips equaled 1% of 0D. weight. The polysulfide sulfur in the drawn-off liquor was unchanged. The chips were then pulped with a regular kraft cooking liquor, containing 17.5% effective alkali as NaOH (on 0D. wood) at a liquor:wood ratio of 35:1. The time from 110C. to C. was 45 minutes, and the time at 170C. was 70 minutes.

For Cook No. 2, the conventional polysulfide cook, a sample of the steamed chips was pulped with a cooking liquor containing 205% effective alkali as NaOH and 1.1% polysulfide sulfur (on D. wood). The sulfidity of the white liquor was 24.2% and the liquor-towood ratio was 3.5: l .0. The time to maximum temperature (170C.) was 90 minutes, and the time at 170C. was 70 minutes.

Cook No. 3 was a conventional kraft cook in which the sample of steamed chips was pulped with a cooking liquor containing 20.5% effective alkali as NaOl-l (based on O.D. wood) with a liquor-to-wood ratio of 4:1. The sulfidity of the liquor was 24.1%. The time to a maximum temperature of 170C. was 90 minutes and the time at that temperature was 75 minutes.

The results of each cook appear in the table below.

The results clearly show that an increase in pulp yield is obtained when using the process of this invention.

The invention as broadly disclosed hereinbefore is an improved process for' increasing continuous pulping yields by impregnating steamed chips with a polysulfide-ion containing solution, the polysulfide-ion containing solution being replenished by white liquor which is sufficiently low in effective alkali that the polysulfide ion is not degraded at impregnation temperatures. Within the scope of this disclosure, a broad range of variations is feasible without departing from the scope of the invention which is defined in the following claims.

What I claim is:

1. A process for increasing the pulp yield of continuously digested lignocellulosic materials consisting of:

a. impregnating said lignocellulosic materials with an aqueous solution containing polysulfide ions at a temperature between C. and C. at an ambient pl-l between 11 and 13,

b. withdrawing the excess polysulfide ion-containing solution,

c. treating said impregnated lignocellulosic material with a solution containing hydroxyl ions whereby the aldehyde end groups of the lignocellulosic materials are oxidized carbonyl groups which are protected against alkaline degradation,

(1. withdrawing at least a portion of said solution containing hydroxyl ions, said solution being at a pH between 10 and 12,

e. preparing said aqueous polysulfide ion-containing solution for use in step (a) from a base solution, said base solution comprising a combination of the withdrawn excess polysulfide ion-containing solution and the withdrawn hydroxyl ion-containing solution by admixing thereto from 0.5% to 2.9% molten sulfur at a temperature between 90C. and 140C. and then,

f. digesting said hydroxyl ion treated impregnated lignocellulosic material in a conventional sulfate cook.

2. The' process as defined in claim 1 further comprising, impregnating said lignocellulosic materials with said aqueous solution containing polysulfide ions at a circulation factor from one to ten.

3. The process as defined in claim 1 wherein said solution contaning hydroxyl ions is kraft white liquor.

4. The process as defined in claim 1 wherein said lignocellulosic materials are steamed pine wood chips.

, 5'1" "@555 PATENT OFFICE CERTiFlCA TE OF CORRECTION PATH: '5 20. 3 74 991 D-fl'ta April 1, 1975 it is -';t;tsaJ "Lit (THE! appt:.-,sa 2:. the JDOV-|Gitiifi(i patent and that said Letters Patent out-acted as shown below Column 3, line 31, "90C. 150C." should read -90C. -l50C. w

! Column 4, line 24, "downwardmoving" should read -downward moving- Column 4, line 25, "draft" should read -kraft.

Column 7, line 19, "maintatin" should read -maintain-- Column 7, line 20, "While" should read -White-- Column 8, line 17, "elasped" should read elapsed- Column 9, line 13, "24. 1%" should read "24. 270".

Column 10, line 24, in claim 1(e) "2. 9%" should read --2. 070

Column 10, line 35, in claim 3, "contaning" should read -containing-.

Signed and Scaled {is twenty-second Day Of July 1975 [SEAL] Arrest.

RUTH C. MASON C. MARSHALL DANN Arresting Office Commissioner afferent: and Trademarks 

1. A PROCESS FOR INCREASING THE PULP YIELD OF CONTINUOUSLY DIGESTED LIGNOCELLULOSIC MATERIALS CONSISTING OF: A. IMPREGNATING SAID LIGNOCELLULOSE MATERIALS WITH AN AQUEOUS SOLUTION CONTAINING POLYSULFIDE IONS AT A TEMPERATURE BETWEEN 90*C. AND 140*C. AT AN AMBIENT PH BETWEEN 11 AND 13, B. WITHDRAWING THE EXCESS POLYSULFIDE ION-CONTAINING SOLUTION. C. TREATING SAID IMPREGNATED LIGNOCELLULOSIC MATERIAL WITH A SOLUTION CONTAINING HYDROXYL IONS WHEREBY THE ALDEHYDE END GROUPS OF THE LIGNOCELLULOSIC MATERIALS ARE OXIDIZED CARBONYL GROUPS WHICH ARE PROTECTED AGAINST ALKALINE DEGRADATION. D. WITHDRAWING AT LEAST A PORTION OF SAID SOLUTION CONTAINING HYDROXYL IONS, SAID SOLUTION BEING AT A PH BETWEEN 10 AND 12, E. PREPARING SAID AQUEOUS POLYSULFIDE ION-CONTAINING SOLUTION FOR USE IN STEP (A) FROM A BASE SOLUTION, SAID BASE SOLUTION COMPRISING A COMBINATION OF THE WITHDRAWN EXCESS POLYSULFIDE ION-CONTAINING SOLUTION AND THE WITHDRAWN HYDROXYL ION-CONTAINING SOLUTION BY ADMIXING THERETO FROM 0.5% TO 2.9% MOLTEN SULFUR AT A TEMPERATURE BETWEEN 90*C. AND 140*C. AND THEN, F. DIGESTING SAID HYDROXYL ION TREATED IMPREGNATED LIGNOCELLULOSIC MATERIAL IN A CONVENTIONAL SULFATE COOK.
 2. The process as defined in claim 1 further comprising, impregnating said lignocellulosic materials with said aqueous solution cOntaining polysulfide ions at a circulation factor from one to ten.
 3. The process as defined in claim 1 wherein said solution contaning hydroxyl ions is kraft white liquor.
 4. The process as defined in claim 1 wherein said lignocellulosic materials are steamed pine wood chips. 